isobar

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A line on a weather map connecting points of equal atmospheric pressure. Also called isopiestic.
2. n. Any of two or more kinds of atoms having the same atomic mass but different atomic numbers.

Century Dictionary and Cyclopedia

1. n. In physical geography, a line connecting places on the surface of the globe at which the barometric pressure is the same. For places not situated at the sea-level, a correction must be applied to each barometric observation corresponding to the elevations of the stations, before the isobar connecting such stations can be drawn. Isobars may be purely imaginary lines; but generally, that the distribution of the pressure may be seen at a glance, they are drawn upon some kind of map or chart of the regions covered by the observations. Isobars may be such as indicate the distribution of barometric pressure at a certain specified day and hour, or they may give the mean pressure for any period of time, as for the entire year or for the summer or winter months. Also called isobarometric line.
2. n. In thermodynamics, a line or curve determined by the equation of state (pv = RT) of a system when p has any constant value; a line of equal pressures.

Wiktionary

1. n. meteorology A line drawn on a map or chart connecting places of equal or constant pressure.
2. n. nuclear physics Either of two nuclides of different elements having the same mass number.
3. n. thermodynamics A set of points or conditions at constant pressure.

GNU Webster's 1913

1. n. (Phys. Geog.) A line connecting or marking places upon the surface of the earth where height of the barometer reduced to sea level is the same either at a given time, or for a certain period (mean height), as for a year; an isopiestic line.

WordNet 3.0

1. n. (meteorology)an isogram connecting points having equal barometric pressure at a given time